1 . Field of the Invention
The present invention relates to belt-driven computer tape cartridges. In particular, the invention relates to a yoke mechanism with rollers for controlling tape tension variations in the cartridge tape.
2 . Description of Related Art
A highly successful elastic belt-driven data cartridge originally was described in U.S. Pat. No. 3,692,255 to von Behren. In that reference, a tape cartridge has tape wrapped convolutely in opposite directions around hubs and guide pins to guide the tape past the tape drive read/write head. An elastic belt wraps pan-way around the tape packs and the hubs, as well as around comer rollers and a drive roller. The belt is moved by the drive through frictional rotation of a drive puck by a motor. Provided adequate tension is maintained in the belt, movement of the belt will simultaneously move the tape. Friction at the comer rollers will differentially stretch the outer surface of the belt, which tensions the tape so that it can interface properly with the read/write heads.
Conventional belt tensioning configurations produce excessive "bow tie," an increase in tape tension which occurs near the end of tape (EOT), or, when the tape moves in the reverse direction, near the beginning of tape (BOT). The bow tie effect becomes apparent upon analysis of tape tension plots. High levels of tension near BOT and EOT stresses and distorts the tape, while increased average tape tension increases tape and read/write head wear, increases drive force, and generally results in less than ideal transfer of tape from one tape pack to the other. Low tape tension may cause increased read errors.
Various roller designs and roller placement configurations have been investigated in an effort to improve tape tension control in belt driven data cartridges with dual tape packs. For example, the tape cartridge described in EP 0466608 uses no corner rollers to stretch the entire belt cross section, but instead uses a single large pack roller to stretch the belt outer surface and control variations in tape tension (such tape cartridges have been manufactured by Societe Anonyme DYPI, a French Corporation, and are well known in the art). As shown in FIGS. 7-8 of the EP '608 patent, after wrapping around a drive roller 5, an elastic belt 8 contacts a first supply tape pack 3, passes between the tape pack 3 and the pack roller 9, wraps around the pack roller, passes between the pack roller and a second take-up tape pack 3', contacts the take-up pack 3', and returns to the drive roller 5. FIGS. 7-8 of the EP '608 patent also show a "watchplate" construction in which a plate 11 positioned parallel to the plane of the baseplate 1 engages the shafts 14,14' of the tape packs to improve shaft stability.
The belt driven cartridge in FIGS. 7-8 of the EP '608 patent develops reasonably good tape tension with little apparent roller drag around the belt path. However, the large pack roller 9 is biased so the belt transfers a large portion of its energy to the take-up pack 3' after the point of initial contact with the tape. An enhanced belt outer surface speed relative to the roller outer surface is difficult to achieve as the belt unwraps from the single large roller 9. The large pack roller 9 in constant contact with both tape packs also prevents rotation of the hubs to remove tape slack if the cartridge is removed from the drive.
FIG. 9 of Japanese Utility Model 63-122966 describes a tension control mechanism with a three roller yoke 50 including two outboard tension control rollers 8a, 8b in contact with the tape packs. A pin 10c of the central roller 8c is not rigidly mounted to the housing 2, but is free to articulate in a slot 51 generally parallel to a line projected through the shafts 5 of the hubs 4. In this design, the wide yoke spreads rollers 8a, 8b apart nearly as far as the hub pins 5, and the rollers cannot be pulled by the belt to press tightly enough against the tape packs to efficiently transfer the belt's excess outer surface speed to the tape. As shown, belt tension would instead force the yoke and the outboard tension control rollers 8a, 8b away from the tape pack. A sudden movement of the yoke pin 10c in the slot 51 would not maintain firm engagement of the belt and tape packs as the packs vary in size. Over the life of the cartridge, buildup of din and debris in slot 51 could prevent smooth operation of the yoke.
As recording densities and tape transport speeds increase, it has become apparent that precise control over tape tension is critical to enhancement of record/playback performance. The tape tension variations, particularly near the beginning and end of tape, produced in conventional data cartridge designs are inadequate to meet projected future recording needs.